What was done
The authors grew plants from seed to maturity in standard perlite bags within climate-controlled greenhouses at Almeria, Spain, during which time the plants were "fertigated" (fertilized and irrigated) via a nutrient-solution drip system that was regulated to maintain the same electrical conductivity in the leached solution draining from the perlite bags of each greenhouse, one of which greenhouses was also supplied with extra CO2 during daylight hours (through outlets below each plant) when the greenhouse side vents were closed or when the roof vent was less than 20% of full opening, resulting in a mean daytime concentration of about 450 ppm around the plants, which was approximately 100 ppm more than the ambient CO2 concentration around the plants in the other greenhouse.

What was learned
The total season-long yield of the cucumber crop was increased by 19% by the extra 100 ppm of CO2 supplied to it during daylight hours, while the overall water use efficiency of the CO2-enriched plants, based on the amount of water supplied to them, was about 40% higher.

What it means
The five Spanish researchers conclude their report by stating that their study "confirms the potential interest of using moderate CO2 enrichment strategies in greenhouses located in areas such as the Mediterranean basin, where the agricultural sector is facing scarce and declining water resources, and needs to drastically reduce the contamination due to fertilizer emission to ensure the sustainability of greenhouse production." And, of course, it demonstrates the significant benefits that may come to open-field agriculture, as the CO2 content of earth's atmosphere continues to rise.